This application claims the priority of Japanese Patent Application No. 2001-290538 filed on Sep. 25, 2001 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electronic endoscope, and more particularly, to signal processing control of an electronic endoscope which reads signals of all pixels of a still image obtained by exposure within a 1 vertical scanning period and forms a 1-frame image.
2. Description of the Related Art
In an electronic endoscope, light from a light source lamp is guided through a light guide to a tip of a scope and the light is irradiated from this scope tip onto an object under observation and an image of this object is taken by an image pickup element such as a CCD (Charge Coupled Device) via an objective optical system. This CCD reads signals (charges) stored every 1-field period one by one and forms moving images. On the other hand, when a still image is formed, the CCD reads signals of all pixels obtained by exposure for a 1 vertical scanning period using a light shielding period in order to reduce influences of motions of the object under observation. A conventional example of the technology for reading signals of all pixels is disclosed in, for example, Japanese Patent Laid-Open No. 9-80322.
FIG. 6 shows an internal configuration of a conventional light source apparatus and this apparatus is provided with a light quantity adjuster 3 in front of a lamp 2 which is turned on by a lighting circuit 1, a light shielding shutter 4 that shields the light from the lamp 2 and a condensing lens 5, and the light from this condensing lens 5 is supplied to an incident end of a light guide 6. There is also a shutter drive circuit (including a motor) 7 that drives the light shielding shutter 4 and this shutter drive circuit 7 shields the light from the lamp for a 1 vertical scanning period, for example, by driving and turning the shutter 4.
FIG. 7 shows the light (exposure) output through the light guide 6 and the reading of a signal from the CCD. For example, a vertical sync signal for a 1/60 sec period shown at a top of FIG. 7 is used. Then, as shown in a middle of FIG. 7, control is performed in such a way that periods L1, L2, . . . during which light is output and a light shielding period C are alternated for every 1 vertical scanning period, and as shown at the bottom of FIG. 7, odd lines 1, 3, 5, . . . of pixel signals (charges) stored in the CCD for the light output period L1 are read in the next period (light shielding period C) and even lines 2, 4, 6, . . . are read in the period after the next period. That is, the light shielding period C is provided to eliminate stored charges in the CCD and allow signals on the even lines to be read.
The signals on the odd lines read from the CCD in this way are processed as odd field signals, while the signals on the even lines are processed as even field signals and it is possible to form moving images or still images by using these field signals. Furthermore, this all pixel reading system can only be executed when the still image is formed, and according to this system, an image is formed by all pixel signals obtained by exposure within 1 vertical scanning period of 1/60 sec, providing an advantage of obtaining a clear image which is not affected by motions of an object under observation, etc.
However, the conventional electronic endoscope needs to include the light shielding shutter 4 to shield light of the lamp 2 and the shutter drive circuit 7 including a motor to drive this shutter 4 as shown in FIG. 6 to create the above-described light shielding period, which involves a problem of complicating the configuration. Furthermore, because of the need to completely shield light in an effective period except a blanking period within 1 vertical scanning period, the conventional electronic endoscope must speedily perform operations of starting driving and stopping the light shielding shutter, which causes inconvenience such as complicating drive control and complicating the drive circuit itself.